The present invention discloses a novel process and novel intermediates toward the preparation of optionally protected 2,5-di-(3xe2x80x2-aminopropyl)pyridines which are useful in the synthesis of xcex1v integrin receptor antagonists.
The present invention provides a novel process for the preparation of optionally amino group-protected 2,5-di-(3xe2x80x2-aminopropyl)pyridines of structural formula I, 
wherein P1 is H2 or a primary amine protecting group. The present invention also provides novel intermediates useful in the disclosed process.
The synthesis of the compound of formula I wherein P1 is H2 was previously disclosed in UK Patent Application GB 2,356,630 (May 30, 2001). In that publication, the 2,5-bis-substituted pyridine ring system was constructed by means of a one-pot double Suzuki cross-coupling of a 2,5-dihalopyridine with a protected allylamine in the presence of 9-BBN and subsequent removal of the primary amine protecting groups.
In the present invention, the compound of formula I wherein P1 is H2 is produced in a highly efficient manner in a total of three chemical steps featuring a one-pot double Sonogashira reaction of a 2,5-dihalopyridine with an optionally protected propargylamine, followed by hydrogenation, and final cleavage of the primary amine protecting groups P1, if required.
The compounds of formula I wherein P1 is a primary amine protecting group are also prepared in an efficient fashion by two consecutive Sonogashira reactions with orthogonally protected propargylamines, followed by hydrogenation, and removal of protecting group P2 on the aminopropyl functionality at the C-5 position of the pyridine ring leaving the aminopropyl group at the C-2 position of the pyridine ring protected with P1.
This invention is concerned with a process for preparing optionally amino group-protected 2,5-di-(3xe2x80x2-aminopropyl)pyridines of structural formula I, wherein P1 is H2 or a primary amine protecting group, and useful intermediates obtained during that process. The process utilizes a double Sonogashira reaction of a 2,5-dihalopyridine with optionally protected propargylamines, hydrogenation, and final removal of the primary amine protecting group, if required. 
The novel process and novel intermediates are illustrated in the following embodiments denoted in Schemes 1 and 2 below. Scheme 1 illustrates the preparation of the compound of formula I wherein P1 is H2. 
Scheme 2 illustrates the preparation of compounds of formula I wherein P1 is a primary amine protecting group. 
Another aspect of the present invention is concerned with an improved process for the preparation of N-formylpropargylamine, a substrate for the Sonogashira reaction disclosed herein.
One aspect of the process of the present invention involves the preparation of the compound of structural formula Ia: 
comprising the steps of:
(a) producing a compound of structural formula II: 
wherein P1 is H2 or a primary amine protecting group, by reacting a 2,5-dihalopyridine with an optionally protected propargylamine of structural formula III: 
in the presence of a palladium catalyst and a base;
(b) producing a compound of structural formula IV: 
by hydrogenating a compound of structural formula II: 
and (c) removing the primary amine protecting groups P1 in a compound of structural formula IV: 
when P1 represents a primary amine protecting group.
The key steps in this first aspect of the process of the present invention include a double Sonogashira reaction of a 2,5-dihalopyridine with an optionally protected propargylamine, hydrogenation, and removal of the primary amine protecting groups, if necessary.
One substrate for the double Sonogashira reaction is an optionally protected propargylamine. In one embodiment of the process of the present invention, the propargylamine is protected as its N-acetyl derivative. This is accomplished by treatment of propargylamine with acetic anhydride or acetyl chloride in a suitable solvent, such as methylene chloride, tetrahydrofuran, toluene, hexane, ethyl acetate, isopropyl acetate, water, lower alkanol, and aqueous lower alkanol, or mixtures thereof.
In a second embodiment of the process of the present invention, the propargylamine is protected as its N-formyl derivative. This is accomplished by treatment of a propargyl halide, a propargyl C1-4 alkylsulfonate, such as propargyl methanesulfonate, propargyl trifluoromethanesulfonate, a propargyl arylsulfonate, such as propargyl benzenesulfonate and propargyl p-toluenesulfonate, or a propargyl di(C1-4 alkyl)phosphate with an alkali metal diformylamide, such as sodium diformylamide, in a suitable organic reaction solvent, such as acetonitrile, tetrahydrofuran, DMF, and the like. One of the two formyl protecting groups in the resulting N,N-diformylpropargylamine is then cleaved by treatment with an inorganic base, such as potassium carbonate, sodium carbonate, potassium hydroxide and sodium hydroxide, in the presence of methanol or ethanol, preferably one to two molar equivalents thereof, to afford N-formylpropargylamine, the substrate for the Sonogashira reaction of the present invention. In one embodiment, An alternative but less economical synthesis of N-formylpropargylamine was described in Bull. Soc. Chim. Fr., 588 (1967). The use of sodium diformylamide as a modified Gabriel reagent for the synthesis of primary amines was described in Synthesis, 122-124 (1990).
Other amine protecting groups may also be used and include t-butyloxycarbonyl (Boc), benzyloxycarbonyl (Cbz), 9-fluorenylmethyloxycarbonyl (FMOC), allyloxycarbonyl (Alloc), phthaloyl, benzoyl, and pivaloyl. Reference is made to T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, 2nd Edition (1991) for a description of other primary amine protecting groups which may be employed in the present process.
The second Sonogashira coupling partner is a 2,5-dihalopyridine. In one embodiment, the 2,5-dihalopyridine is 2,5-dibromopyridine. However, 2,5-dichloropyridine, 2,5-diiodopyridine, or a mixed 2,5-dihalopyridine, such as 2-chloro-5-bromo-pyridine, may also be employed in the reaction.
The double Sonogashira reaction of a 2,5-dihalopyridine is effected with the optionally protected propargylamine in the presence of a palladium catalyst and a base.
The Sonogashira reaction is optionally carried out in a suitable organic solvent, such as THF, benzene, toluene, dioxane, acetonitrile, aqueous acetonitrile, DME, DMSO, DMF, DMAC, and NMP, or a mixture of these solvents, such as THF/DMF.
Palladium catalysts which may be used in the Sonogashira reaction include a palladium alkanoate, a palladium acetonate, a palladium halide, a palladium halide complex, a palladium-dibenzylidene acetone complex, and a triarylphosphine palladium complex. More specifically, the palladium catalyst is selected from the group consisting of Pd(II) acetate, Pd(II) acetylacetonate, Pd(0)bis-dibenzylidene acetone (xe2x80x9cdbaxe2x80x9d), Pd(II) bromide, Pd(II) chloride, Pd(II) iodide, Pd(II) sulfate, Pd(II) trifluoroacetate, Pd(II)Cl2(CH3CN)2, Pd2(dba)3, Pd(II)(dppf)Cl2, Pd(II)Cl2(PPh3)2, Pd(PPh3)4, and Pd(II)Cl2(PhCN)2. In one embodiment the palladium catalyst is Pd(II)Cl2(PPh3)2.
Bases which may be employed in the process of the present invention include organic aliphatic amines, such as triethylamine, diethylamine, diisopropylamine, diisopropylethylamine, n-butylamine, t-butylamine, 1,4-diazabicyclo[2.2.2]octane (DABCO), and quinuclidine, and organic aromatic amines, such as pyridine and 4-dimethylaminopyridine (DMAP). In one embodiment, the organic amine may serve as the reaction solvent as well as the base. In a class of this embodiment, the organic amine is diisopropylamine. Inorganic bases, such as potassium carbonate, may also be used in place of the organic amine.
The reaction is performed at a temperature range of about 10xc2x0 C. to about 120xc2x0 C. In another embodiment, the optionally protected propargylamine is used in an amount of about 2 to 3 molar equivalents of the 2,5-dihalopyridine.
In another embodiment of the process of the present invention, the Sonogashira reaction is carried out in the presence of a copper, zinc, or zirconium reagent. The addition of a copper, zinc, or zirconium reagent accelerates the rate of the coupling reaction. In one embodiment, the copper reagent is copper metal, a copper(I) halide, such as CuCl, CuBr, and CuI, or a copper(II) halide, such as CuCl2, CuBr2, and CuI2. In a class of this embodiment, the copper reagent is CuBr or CuI. However, other copper(I) and copper(II) salts may also be utilized to accelerate the coupling reaction, such as copper(I) acetate and copper(I) cyanide. In another embodiment, the zinc reagent is zinc metal or a zinc salt, such as ZnCl2, ZnBr2, ZnI2, or Zn(OTf)2. In a further embodiment, the zirconium reagent is bis(cyclopentadienyl)zirconium dichloride.
The double Sonogashira reaction product is a compound of structural formula II: 
wherein P1 is H2 or a primary amine protecting group. The next step of the process is hydrogenation of the two triple bonds in structure II to afford a compound of structural formula IV. The hydrogenation is typically carried out in a solvent, such as THF, lower alkanol, such as methanol and ethanol, or aqueous lower alkanol, such as aqueous methanol and aqueous ethanol, at a hydrogen gas pressure of about 1 atmosphere to about 120 p.s.i., in the presence of a noble metal catalyst, such as Raney nickel, Pd/C, Rh/C, Ru/C, Pd/Al2O3, PtO2, Pt/C, Pt/Al2O3, Rh/Al2O3, and Ru/Al2O3.
The last step of this aspect of the process of the present invention is the removal of the protecting groups P1 in intermediate IV to generate the compound of structural formula I. When the primary amine protecting group is acetyl or formyl, it may be cleaved by treatment with aqueous acid or base preferably at an elevated temperature. In one embodiment, the acetyl or formyl group is cleaved by heating in aqueous hydrochloric acid. When the primary amine protecting group is t-butyloxycarbonyl, it may be cleaved by treatment with trifluoroacetic acid, sulfuric acid, HCl in ethyl acetate, HCl in diethyl ether, or HCl in dioxane. When the primary amine protecting group is benzyloxycarbonyl, reduction of the two triple bonds in structure II and removal of the P1 protecting groups may be effected in one step, such as by catalytic hydrogenation. Other protecting groups can be removed by standard literature conditions, such as those found in T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, 2nd Edition (1991) and P. J. Kocienski, Protecting Groups, Georg Thieme Verlag, New York, 1994.
Another aspect of the present invention concerns a process of preparing a compound of structural formula I, wherein P1 is a primary amine protecting group, 
which comprises the steps of:
(a) producing a compound of structural formula V: 
wherein Y is Cl, Br, or I; by reacting a 2,5-dihalopyridine with a protected propargylamine of structural formula III: 
in the presence of a palladium catalyst and a base;
(b) producing a compound of structural formula VI: 
wherein P1 and P2 are orthogonal primary amine protecting groups, by reacting a compound of structural formula V: 
with a protected propargylamine of structural formula VII: 
in the presence of a palladium catalyst and a base;
(c) producing a compound of structural formula VIII: 
by hydrogenating a compound of structural formula VI: 
and (d) selectively removing the primary amine protecting P2 in a compound of structural formula VIII: 
The key steps in this second aspect of the process of the present invention to prepare compounds of structural formula I involve two consecutive Sonogashira coupling reactions of a 2,5-dihalopyridine with two different orthogonally protected propargylamines of structural formulae III and VII. The first Sonogashira coupling reaction takes place at the more reactive C-2 position of the pyridine ring to afford a compound of structural formula V. The second Sonogashira coupling reaction takes place at the C-5 position of the pyridine ring to afford a compound of structural formula VI. The two consecutive Sonogashira coupling reactions may be performed in a single pot by first reacting the 2,5-dihalopyridine with a protected propargylamine of structural formula III in the presence of a palladium catalyst and a base and allowing the reaction to go to completion and subsequently adding a second protected propargylamine of structural formula VII and allowing the reaction to go to completion. Alternatively, the intermediate compound of structural formula V may be isolated after the first coupling reaction with III and then subjected to a second Sonogashira coupling reaction with a protected propargylamine of structural formula VII. The two triple bonds in the compound of structural formula VI are then reduced by hydrogenation to afford a compound of structural formula VIII. The final step in the process involves the selective removal of the protecting group P2 in the compound of structural formula VIII without removing protecting group P1 to afford a compound of structural formula I. Protecting groups P1 and P2 are chosen such that P2 can be selectively cleaved under reaction conditions that do not affect P1. An example of such an orthogonal pair of protecting groups is provided by P1 as t-butyloxycarbonyl (t-Boc) and P2 as benzyloxycarbonyl (Cbz) wherein the benzyl carbamate can be selectively cleaved under hydrogenolytic conditions without affecting the t-butyl carbamate group. These hydrogenolytic conditions will also reduce the triple bonds in a compound of formula VI such that the desired compound of formula I is obtained in a single step from intermediate VI.
The first coupling reaction with intermediate III is performed at a temperature range of about 10xc2x0 C. to about 30xc2x0 C. The P1 protected propargylamine III is used in an amount of about 0.95 to about 1.2 molar equivalents of the 2,5-dihalopyridine. The second coupling reaction with intermediate VII is performed at a temperature range of about 10xc2x0 C. to about 90xc2x0 C. The P2 protected propargylamine VII is used in an amount of about 0.95 to about 2.0 molar equivalents of the 2,5-dihalopyridine.
In one embodiment of this second aspect of the present invention, the Sonogashira coupling reaction is carried out in the presence of a copper reagent, such as copper(I) bromide and copper(I) iodide, or a zinc reagent, such as ZnCl2 and ZnBr2.
Compounds of structural formula I of the present invention can be converted into 3-(5,6,7,8-tetrahydro-[1,8]-naphthyridin-2-yl)-propylamine (IX) as described in GB 2,356,630 (May 30, 2001). Compound IX is a useful intermediate in the preparation of xcex1v integrin receptor antagonists as described in WO 01/34602. 
Another aspect of the present invention is concerned with novel compounds which are useful intermediates in the preparation of the compounds of structural formula I. One embodiment of this aspect of the present invention comprises compounds of the structural formula: 
wherein P1 and P2 are each independently selected from the group consisting of hydrogen, acetyl, formyl, benzoyl, pivaloyl, phthaloyl, t-butyloxycarbonyl, benzyloxycarbonyl, allyloxycarbonyl, and 9-fluorenylmethyloxycarbonyl. In one embodiment of the novel compounds, both P1 and P2 represent acetyl, formyl, or benzyloxycarbonyl. In a second embodiment, P1 is t-butyloxycarbonyl and P2 is benzyloxycarbonyl.
A second embodiment of this aspect of the present invention comprises compounds of the structural formula: 
wherein Y is Cl, Br, or I and P1 is selected from the group consisting of acetyl, formyl, benzoyl, pivaloyl, phthaloyl, t-butyloxycarbonyl, benzyloxycarbonyl, allyloxycarbonyl, and 9-fluorenylmethyloxycarbonyl.
Abbreviations: AcOH is acetic acid; BuLi is n-butyl lithium; CH2Cl2 is dichloromethane; DMAC is N,N-dimethylacetamide; DME is 1,2-dimethoxyethane; DMF is N,N-dimethylformamide; DMPU is 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; DMSO is dimethyl sulfoxide; EtOAc is ethyl acetate; Et3N is triethylamine; K2CO3 is potassium carbonate; MgSO4 is magnesium sulfate; MTBE is methyl t-butyl ether; NMR is nuclear magnetic resonance; Na2CO3 is sodium carbonate; NaHCO3 is sodium hydrogencarbonate; and THF is tetrahydrofuran.
By halide is meant chloride, bromide, or iodide.
By lower alkanol is meant a C1-5 linear or branched-chain alkyl alcohol, such as methanol, ethanol, isopropanol, and 1-butanol.
By xe2x80x9corthogonal protectionxe2x80x9d is meant two different protecting groups for each primary amino group on compounds of structural formulae VI and VIII, the removal of each of which is accomplished in any order with reagents and conditions which do not affect the other protecting group. An example of such orthogonal protection is benzyloxycarbonyl for one primary amino group and t-butyloxycarbonyl for the other primary amino group.
Representative experimental procedures utilizing the novel process of the present invention are detailed below. They are given for purposes of illustration only and are not intended to limit the process of the present invention to the specific conditions given for making the exemplified compounds.
Preparation of N-Protected Propargylamines:
N-Formylpropargylamine:
Step A: Sodium diformylamide
HCONH2+NaOMexe2x86x92NaN(CHO)2 
To a solution of sodium methoxide in methanol (25.4 wt %, 362 g) and formamide (168 g) was added 1,2-dimethoxyethane (200 mL). The mixture was distilled and additional 1,2-dimethoxyethane (850 mL) was added at a rate such that by the end of the addition (3.5 h) a total of 1200 mL of distillate had been collected. The mixture was cooled to 20-25xc2x0 C. and filtered. The cake was dried to afford 158 g of sodium diformylamide.
1H NMR (DMSO-d6; 400 MHz): xcex4 8.96 (s).
13C NMR (DMSO-d6; 101 MHz): xcex4 180.6.
Step B: N-formylpropargylamine 
To a slurry of sodium diformylamide (157 g) in acetonitrile (1 L) was added propargyl bromide (80 wt % in toluene, 149 mL), and the mixture was heated to reflux for 11 h. The mixture was cooled to 20-25xc2x0 C, methanol (123 mL) and potassium carbonate (49 g) were added, and the mixture was stirred at ambient temperature for 1 h. The mixture was filtered, and the filtrate was concentrated at reduced pressure to give an oil. The oil was redissolved in methylene dichloride (200 mL), the solution was filtered through a pad of silica gel (100 g), and the filtrate was concentrated at reduced pressure to afford crystalline N-formylpropargylamine (112 g).
1H NMR (CDCl3; 400 MHz): major rotamer xcex4 8.13 (s, 1H), 6.94 (br, 1H), 4.02 (ddd, J=5.6, 2.6, 0.8 Hz, 2H), 2.23 (t, J=2.6 Hz, 1H); minor rotamer xcex4 8.09 (d, J=12.1 Hz, 1 H), 6.51 (br, 1 H), 3.97 (dd, J=6.0 and 2.6 Hz, 2 H), 2.34 (t, J=2.6 Hz, 1 H).
13C NMR (CDCl3; 101 MHz): major rotamer xcex4 161.4, 78.9, 71.3, and 27.4; minor rotamer xcex4 164.7, 78.7, 72.8, and 31.2.
N-Acetylpropargylamine: 
To a solution of propargylamine (20.0 g) and triethylamine (69.7 mL) in ethyl acetate (500 mL) was slowly added acetyl chloride (33.6 mL), maintaining the inner temperature below 30xc2x0 C. The mixture was aged at ambient temperature overnight. Water (130 mL) was added to the mixture. The resulting organic layer was separated. The aqueous layer was extracted with ethyl acetate (100 mL) then dichloromethane (100 mLxc3x976) after saturating the aqueous layer with sodium chloride. The organic layers were combined, dried over magnesium sulfate, and concentrated under reduced pressure. The residue was crystallized from ethyl acetate (50 mL). The crystals were collected by filtration, washed with ethyl acetate, and dried to give 22.1 g of the title compound. The filtrate and wash were combined and purified by silica gel column chromatography with ethyl acetate as eluant. The fractions containing the title compound were combined, concentrated, and crystallized from MTBE to give an additional 9.33 g of the title compound.
1H NMR (CDCl3; 400 MHz): xcex4 6.09 (broad s, 1H), 4.03 (dd, J=5.2 and 2.8 Hz, 2H), 2.22 (t, J=2.8 Hz, 1H), and 2.01 (s, 3H).
13C NMR (CDCl3; 101 MHz): xcex4 169.9, 79.5, 71.4, 29.2, and 22.9.
N-Cbz-propargylamine: 
To a solution of propargylamine (20 g) and triethylamine (60.7 mL) in ethyl acetate (500 mL) was added benzyloxycarbonyl chloride (71.7 g) slowly while maintaining the reaction temperature between xe2x88x925xc2x0 C. and +5xc2x0 C. The mixture was then stirred at room temperature overnight. The mixture was washed sequentially with 1.5 M HCl (200 mL), 5% NaHCO3 in water (200 mL), and saturated aqueous sodium chloride (100 mL). The solvent was switched to hexane while the desired product precipitated as colorless crystals (60.1 g).
1H NMR (CDCl3; 400 MHz): xcex4 7.40xe2x88x927.30 (m, 5H), 5.14 (s, 2H), 5.01 (broad s, 1H), 4.00 (m, 2H), and 2.25 (t, J=2.4 Hz, 1H).
13C NMR (CDCl3; 101 MHz): xcex4 155.9, 136.2, 128.5, 128.22, 128.17, 79.6, 71.6, 67.1 and 30.9.
N-Boc-propargylamine: 
To di-tert-butyl dicarbonate (25.3 mL) was added propargylamine (7.11 mL) under ice-cooling. The mixture was aged at ambient temperature overnight. The mixture was concentrated under reduced pressure, and the residue was diluted with hexane and further concentrated under reduced pressure. The resulting crystalline residue was triturated with hexanes, collected by filtration, and dried to give 12.5 g of the title compound.
1H NMR (CDCl3; 400 MHz): xcex4 4.79 (broad s, 1H), 3.91 (broad s, 2H), 2.21 (t, J=2.6 Hz, 1H), and 1.44 (s, 9H).
13C NMR (CDCl3; 101 MHz): xcex4 155.2, 80.1, 80.0, 71.2, 30.3, and 28.3.